01 -
Spread mayo on outside of both bread slices, mustard on inside of one slice. Layer cheeses on mustard side.
02 -
Heat skillet over medium-low heat, melt butter to coat.
03 -
Cook covered 2-3 minutes until golden brown.
04 -
Flip, cook covered 1-3 minutes until golden and cheese melts. Reduce heat if browning too fast.